1. Field of the Invention
The present invention relates to an optical recording medium so-called as a next-generation DVD (digital versatile disc), and in particular, to an optical recording medium having two or more information layers made of a phase change material and a recording film material for the optical recording medium.
2. Description of the Related Art
A Blu-ray (registered trademark) Disc (hereinafter referred to as a BD) which uses an optical system where the wavelength of the recording and reproduction laser is 405 nm (blue) and the numerical aperture NA of the objective lens is 0.85 (λ/NA≦650 nm) has been proposed as a next-generation DVD.
In an optical disc drive for such a BD, as a standard in reproduction, the reproduction light is modulated at a High Frequency Modulation being a few hundred MHz in an optical pickup using a single mode oscillation semiconductor laser diode in order to reduce laser noise during signal reproduction.
The general conditions of High Frequency Modulation are determined to be a frequency in a range of from 300 MHz to 500 MHz, a ratio in a range of from 3.0 to 8.0, and a pulse width in a range of from 200 to 400 psec.
In the BD, on the other hand, a rewritable optical recording medium having two information layers on one side is proposed. In the dual-layer optical recording medium, an L1 layer except for an information layer (L0 layer), which is the farthest away from a light incident surface, is translucent in order to allow recording and reproduction light to pass therethrough.
Since the amount of light reflected from such a translucent L1 layer during reproduction is small, it is necessary to increase laser power used during reproduction. Also, the BD uses laser light with a short wavelength and the resultant spot size is small when compared with that of a conventional DVD, so that the energy density at the laser spot becomes extremely high.
Accordingly, the next-generation DVD with two or more layers has a problem in that the irradiation of laser light with a high reproduction power reduces the reproduction durability of the recording signal because, in particular, the layer(s) excluding the L0 layer exhibit low radiation performance.
In particular, the rewritable phase change optical recording medium has a problem in that the irradiation of laser light with a high reproduction power during reproduction crystallizes an amorphous mark (being a recording signal) and tends to erase the recording signal. This problem is particularly notable in a phase change material such as an Sb-based eutectic system having Sb as the main composition.
Thus, it has been difficult to date to use an Sb-based eutectic material in the recording film of a translucent information layer of a rewritable multilayer optical recording medium of a BD.
Furthermore, when the Sb-based eutectic material is used in the recording film of the translucent information layer of a rewritable multilayer BD, as described above, there is a problem in that the margin for the recording strategy becomes narrow as follows.
The Sb-based eutectic material can easily increase the crystallization rate of the recording film. When forming the amorphous mark using recording laser light, however, a high cooling rate is necessary. If the cooling rate is insufficient, recrystallization occurs during cooling which occurs after the melting of the recording film so that the amorphous mark is formed in an unsatisfactory manner.
Therefore, it is necessary to either make the structure of the optical recording medium capable of being rapidly cooled or narrow the pulse width used in the recording strategy for forming the amorphous mark in order to prevent the occurrence of any residual heat during recording.
In the translucent information layer of a dual-layer BD, however, the recording film or reflective film has to be thin enough to allow the recording and reproduction light to pass therethrough. Thus, the dispersal of any residual heat is insufficient and the slow cooling structure of the layer when compared with the information layer (being the L0 layer) of total reflection means that an amorphous mark is not satisfactorily formed.
Furthermore, since it takes approximately 1.5 nsec to 3 nsec of rise and fall time to emit laser light, even if the pulse width of the laser pulse is set to be shorter than the aforementioned emission time, the laser light cannot be emitted. Thus, it is essential that amorphous marks are formed on the recording film with a recording pulse width that is wider than that detailed above.
However, since the Sb-based eutectic material requires a high cooling rate, as described above, the recrystallization occurs when amorphous marks are formed with a recording strategy having a wide pulse width, so that it is difficult to correctly form amorphous marks. In Japanese Patent No. 3899770, they say that it is possible to fit the optical recording media to wide range of linear velocity and to make a amorphous mark stable by using the SbTe-based eutectic material as the recording film. But the invention mentioned in Japanese Patent No. 3899770 could hardly substantialize reproduction durability at high laser power during reproduction using an optical system of λ/NA≦650 nm.